"deja-dup --restore-missing" takes >10min to read backup data

I back up my $HOME using Deja-Dup 34.2 in Ubuntu 16.04. Today I tried "deja-dup --restore-missing $HOME" to restore an accidentally deleted file and a window popped up telling me that it would read the files of the last backup, the last week (which took ~5min) and now it is reading the last month.

From the time it took for the last week of backups I would estimate it requires about 30-40s per day of backups.
And I have six months worth of backups on my NAS.

Meanwhile, one of my 4 CPUs is maxed out (100%).

1. What is taking so long to read the backup? There is no network traffic so I assume it's just reading an index, not importing the whole multi-GB backup.
2. What is taking so much CPU reading the backup?
3. Why is it only using one CPU? ;-)

Thank you!